Thanks Calder - did the Rum flavor come through with those measurements? I'd like it to be a notch or two above "subtle".

At 500 ml per bottle, you are increasing the abv by about 1%, so be careful how much you add. It will also reduce the FG of the beer making it drier ane thinner to taste.

I knew the rum was there, but it was not obvious. Using Bourbon to the same amount, it has much more presence.

One way to find out is to try it in a beer. Take any beer (preferably one similar to the one you are brewing, but any would suffice if you realize the differences), and add 10 ml to it in the glass and see what effect it has. 10 ml is easy to measure out, it is 2 teaspoons full, or maybe you have one of those medicine cups that come with Nyquil and the like.